The present invention generally relates to a chemical mechanical polishing apparatus and more particularly, relates to a sweeping slurry dispenser for use in a chemical mechanical polishing apparatus which is capable of spreading a slurry solution more uniformly on the surface of a polishing pad.
Apparatus for polishing thin, flat semi-conductor wafers is well not in the art. Such apparatus normally includes a polishing head which carries a membrane for engaging and forcing a semi-conductor wafer against a wetted polishing surface, such as a polishing pad. Either the pad, or the polishing head is rotated and oscillates the wafer over the polishing surface. The polishing head is forced downwardly onto the polishing surface by a pressurized air system or, similar arrangement. The downward force pressing the polishing head against the polishing surface can be adjusted as desired. The polishing head is typically mounted on an elongated pivoting carrier arm, which can move the pressure head between several operative positions. In one operative position, the carrier arm positions a wafer mounted on the pressure head in contact with the polishing pad. In order to remove the wafer from contact with the polishing surface, the carrier arm is first pivoted upwardly to lift the pressure head and wafer from the polishing surface. The carrier arm is then pivoted laterally to move the pressure head and wafer carried by the pressure head to an auxiliary wafer processing station. The auxiliary processing station may include, for example, a station for cleaning the wafer and/or polishing head; a wafer unload station; or, a wafer load station.
More recently, chemical-mechanical polishing (CMP) apparatus has been employed in combination with a pneumatically actuated polishing head. CMP apparatus is used primarily for polishing the front face or device side of a semiconductor wafer during the fabrication of semiconductor devices on the wafer. A wafer is xe2x80x9cplanarizedxe2x80x9d or smoothed one or more times during a fabrication process in order for the top surface of the wafer to be as flat as possible. A wafer is polished by being placed on a carrier and pressed face down onto a polishing pad covered with a slurry of colloidal silica or alumina in de-ionized water.
A perspective view of a typical CMP apparatus is shown in FIG. 1A. The CMP apparatus 10 consists of a controlled mini-environment 12 and a control panel section 14. In the controlled mini-environment 12, typically four spindles 16, 18, 20, and 22 are provided (the fourth spindle 22 is not shown in FIG. 1a) which are mounted on a cross-head 24. On the bottom of each spindle, for instance, under the spindle 16, a polishing head 26 is mounted and rotated by a motor (not shown). A substrate such as a wafer is mounted on the polishing head 26 with the surface to be polished mounted in a face-down position (not shown). During a polishing operation, the polishing head 26 is moved longitudinally along the spindle 16 in a linear motion across the surface of a polishing pad 28. As shown in FIG. 1A, the polishing pad 28 is mounted on a polishing disc 30 rotated by a motor (not shown) in a direction opposite to the rotational direction of the polishing head 26.
Also shown in FIG. 1A is a conditioner arm 32 which is equipped with a rotating conditioner disc 34. The conditioner arm 332 pivots on its base 36 for the in-situ conditioning of the pad 38 during polishing. While three stations each equipped with a polishing pad 28, 38 and 40 are shown, the fourth station is a head clean load/unload (HCLU) station utilized for the loading and unloading of wafers into and out of the polishing head. After a wafer is mounted into a polishing head in the fourth head cleaning load/unload station, the cross head 24 rotates 90xc2x0 clockwise to move the wafer just loaded into a polishing position, i.e., over the polishing pad 28. Simultaneously, a polished wafer mounted on spindle 20 is moved into the head clean load/unload station for unloading.
A cross-sectional view of a polishing station 42 is shown in FIGS. 1B and 1C. As shown in FIG. 1B, a rotating polishing head 26 which holds a wafer 44 is pressed onto an oppositely rotating polishing pad 28 mounted on a polishing disc 30 by adhesive means. The polishing pad 28 is pressed against the wafer surface 46 at a predetermined pressure. During polishing, a slurry 48 is dispensed in droplets onto the surface of the polishing pad 28 to effectuate the chemical mechanical removal of materials from the wafer surface 46.
An enlarged cross-sectional representation of the polishing action which results form a combination of chemical and mechanical effects is shown in FIG. 1C. The CMP method can be used to provide a planner surface on dielectric layers, on deep and shallow trenches that are filled with polysilicon or oxide, and on various metal films. A possible mechanism for the CMP process involves the formation of a chemically altered layer at the surface of the material being polished. The layer is mechanically removed from the underlying bulk material. An outer layer is then regrown on the surface while the process is repeated again. For instance, in metal polishing, a metal oxide layer can be formed and removed repeatedly.
During a CMP process, a large volume of a slurry composition is dispensed. The slurry composition and the pressure applied between the wafer surface and the polishing pad determine the rate of polishing or material removal from the wafer surface. The chemistry of the slurry composition plays an important role in the polishing rate of the CMP process. For instance, when polishing oxide films, the rate of removal is twice as fast in a slurry that has a pH of 11 than with a slurry that has a pH of 7. The hardness of the polishing particles contained in the slurry composition should be about the same as the hardness of the film to be removed to avoid damaging the film. A slurry composition typically consists of an abrasive component, i.e, hard particles and components that chemically react with the surface of the substrate. For instance, a typical oxide polishing slurry composition consists of a colloidal suspension of oxide particles with an average size of 30 nm suspended in an alkali solution at a pH larger than 10. A polishing rate of about 120 nm/min can be achieved by using this slurry composition. Other abrasive components such as ceria suspensions may also be used for glass polishing where large amounts of silicon oxide must be removed. Ceria suspensions act as both the mechanical and the chemical agent in the slurry for achieving high polishing rates, i.e, larger than 500 nm/min. While ceria particles in the slurry composition remove silicon oxide at a higher rate than do silica, silica is still preferred because smoother surfaces can be produced. Other abrasive components, such as alumina (Al3O2)may also be used in the slurry composition.
The polishing pad 28 is a consumable item used in a semiconductor wafer fabrication process. Under normal wafer fabrication conditions, the polishing pad is replaced after about 12 hours of usage. Polishing pads may be hard, incompressible pads or soft pads. For oxide polishing, hard and stiffer pads are generally used to achieve planarity. Softer pads are generally used in other polishing processes to achieve improved uniformity and smooth surface. The hard pads and the soft pads may also be combined in an arrangement of stacked pads for customized applications.
Referring now to FIG. 2, wherein a perspective view of a CMP polishing station 42 is shown. The polishing station 42 consists of a conditioning head 52, a polishing pad 28, and a slurry delivery arm 54 positioned over the polishing pad. The slurry delivery arm 54 is equipped with slurry dispensing nozzles 62 which are used for dispensing a slurry solution on the top surface 60 of the polishing pad 56. Surface grooves 64 are further provided in the top surface 60 to facilitate even distribution of the slurry solution and to help entrapping undesirable particles that are generated by coagulated slurry solution or any other foreign particles which have fallen on top of the polishing pad during a polishing process. The surface grooves 64 while serving an important function of distributing the slurry also presents a processing problem when the pad surface 60 gradually worn out after successive use.
The slurry delivery arm 54 shown in FIG. 2 delivers a slurry solution to the polishing pad 28 in a stationary manner. The distribution of the slurry solution over the top surface of the polishing pad depends on the rotation of the pad. Since the slurry solution is usually dispensed at the center of the polishing platen, i.e., at the center of the polishing pad, it is difficult to spread evenly the slurry solution over the pad surface by the rotation of the pad. As a result, the amount of slurry at the edge of the polishing pad is always less than that in the center region of the pad. This leads to a higher removal rate at the center of the pad when compared to the edge portion of the pad. And furthermore, a higher polishing noise level is made during the polishing process. The problem is more severe in the newly designed polishing pads which have deeper surface grooves than the older pads, it thus becomes more difficult to spread the slurry solution uniformly on the polishing pad.
It is therefore an object of the present invention to provide a slurry dispenser for chemical mechanical polishing that does not have the drawbacks or shortcomings of the conventional slurry dispensing arms.
It is another object of the present invention to provide a slurry dispenser for chemical mechanical polishing that is capable of spreading a slurry solution more uniformly on top of a polishing pad.
It is a further object of the present invention to provide a sweeping slurry dispenser for chemical mechanical polishing that moves a slurry dispensing nozzle in an arcuate path on top of a polishing pad.
It is still another object of the present invention to provide a sweeping slurry dispenser for chemical mechanical polishing that moves a slurry dispensing nozzle in a half-circular pattern on top of a polishing pad.
It is still another object of the present invention to provide a sweeping slurry dispenser for chemical mechanical polishing by utilizing a circular dispensing wheel driven by an oval-shaped drive wheel.
It is yet another object of the present invention to provide a sweeping slurry dispenser for chemical mechanical polishing wherein the slurry dispensing nozzle sweeps in an arcuate pattern on top of a polishing pad during the slurry dispensing process.
It is still another further object of the present invention to provide a sweeping slurry dispenser for chemical mechanical polishing wherein a cam and molter arrangement is used to change a circular motion of a rotational molter to a side-to-side motion of the slurry dispensing nozzle.
It is yet another further object of the present invention to provide a sweeping slurry dispenser in a chemical mechanical polishing apparatus wherein the slurry dispensing nozzle sweeps between an edge of the polishing pad to a center of the polishing pad to provide a uniform distribution of slurry on the pad.
In accordance with the present invention, a sweeping slurry dispenser including a sweeping dispensing nozzle is used in a chemical mechanical polishing apparatus to uniformly distribute a slurry solution on top of a polishing pad.
In a preferred embodiment, a sweeping slurry dispenser in a chemical mechanical polishing apparatus can be provided which includes a drive wheel of oval shape that turns on a fixed center axis, the wheel has a first diameter and a second diameter perpendicular to each other, the first diameter is at least 50% larger than the second diameter; a dispenser wheel of circular shape that turns on a fixed center axis when driven by a push arm with an first end pivotally engaging a shaft mounted on an outer periphery of the dispenser wheel; a push arm of elongated shape that has a hollow center slot therein for engaging a guide pin fixedly attached to the CMP apparatus, a first end fixedly attached to the outer periphery of the dispenser wheel in a distal second end equipped with a roller for rollingly engaging in outer surface of the oval shaped drive wheel, the push arm is further equipped with a spring attached between the guide pin and the shaft on the dispenser wheel to facilitate the turning of the dispenser wheel; a motor means for rotating the drives wheel on the fixed center axis at a preset rotational speed; and a slurry dispensing nozzle attached to the bottom surface of the drive wheel for dispensing a slurry solution in a arcuate path when the dispenser wheel is turned by the drive wheel.
In the sweeping slurry dispenser for a CMP apparatus, the first diameter is between about 50% and about 900% larger than the second diameter, a ratio between the first diameter and the second diameter may be between about 2:1 and about 8:1. The drive wheel may have a thickness sufficiently large for the roller attached to the push arm to roll on an edge portion of the drive wheel. The spring is in a fully extended state when the shaft for fixing the position of the first end of the push arm is in a 3 o""clock or in a 9 o""clock position. The spring is in a fully compressed state when the shaft for fixing the position of the first end of the push arm is in a 6 o""clock position. The slurry dispensing nozzle traverses in a half-circular path when the dispenser wheel is turned by the drive wheel. The slurry dispensing nozzle may further traverse in a path that is substantially similar to a path of a polishing head that holds a wafer to be polished therein. The present rotational speed is between about 1 RPM and about 60 RPM.
In the sweeping slurry dispenser for use in a CMP apparatus, the drive wheel may be fabricated of aluminum, and has a first diameter of about 6 inches and a second diameter of about 2 inches. The dispenser wheel may have a diameter between about 4 inches and about 12 inches, and may be fabricated of aluminum. The push arm may have a length between about 4 inches and about 12 inches and may be fabricated of aluminum.